Why is titanium alloy difficult to work?
1. physical phenomena of titanium processing
The cutting power of titanium alloys is only slightly higher than that of steels of the same hardness, but the physical phenomena of machining titanium alloys are much more complex than those of steels, making the machining of titanium alloys extremely difficult.
The thermal conductivity of most titanium alloys is very low, only 1/7 of steel and 1/16 of aluminum, therefore, the heat generated during the cutting of titanium alloys is not quickly transferred to the workpiece or taken away by the chips, but concentrated in the cutting area, the resulting temperature can be as high as 1 000 ℃ or more, so that the cutting edge of the tool quickly wear, chipping and chip tumor generation, the rapid emergence of worn edges, but also make the cutting area to generate more heat, further reducing tool life.
The high temperatures generated during the cutting process also destroy the surface integrity of the titanium alloy part, leading to a reduction in the geometric accuracy of the part and a work-hardening phenomenon that severely reduces its fatigue strength.
The elasticity of titanium alloys can be beneficial to part performance, but during the cutting process, the elastic deformation of the workpiece is an important cause of vibration. Cutting pressure causes the "elastic" workpiece to leave the tool and rebound, which causes more friction between the tool and the workpiece than cutting action. The friction process also generates heat, exacerbating the problem of poor thermal conductivity of titanium alloys.
This problem is exacerbated when machining deformable parts, such as thin-walled or ring-shaped parts, and machining thin-walled titanium parts to the desired dimensional accuracy is not an easy task. As the workpiece material is pushed apart by the tool, the local deformation of the thin-walled part exceeds the elastic range, resulting in plastic deformation and a significant increase in material strength and hardness at the cutting point. At this point, machining at the originally established cutting speed becomes too high, further resulting in rapid tool wear.
"Heat" is the "culprit" of titanium alloy difficult to process!
2. Process know-how for machining titanium alloys
Based on the understanding of the mechanism of titanium alloy processing, together with past experience, the main process tips for processing titanium alloy are as follows：
(1) Adopt inserts with positive-angle geometry to reduce cutting force, cutting heat, and workpiece deformation.
(2) Maintain a constant feed to avoid hardening of the workpiece, the tool should always be in the feed state during the cutting process, and the radial eating ae should be 30% of the radius when milling.
(3) Adopt high-pressure, high-flow cutting fluid to ensure the thermal stability of the machining process and prevent surface degradation and tool damage caused by high temperature.
(4) Keep the cutting edge of the blade sharp, a dull tool is the cause of heat buildup and wear, which can lead to tool failure.
(5) Machine the titanium alloy in its softest state possible, as the hardened material becomes more difficult to machine and heat treatment increases the strength of the material and increases insert wear.
(6) Use large tip radii or chamfers to cut in, putting as much of the cutting edge into the cut as possible. This reduces the cutting force and heat at each point and prevents local breakage. When milling titanium, cutting speed has the greatest effect on tool life vc of all cutting parameters, followed by radial tool eating (milling depth) ae.
3. Solve titanium machining problems from the beginning of the blade.
Insert groove wear that occurs during titanium machining is localized behind and in front along the depth of cut and is often caused by a hardened layer left over from earlier machining. Chemical reactions and diffusion between the tool and the workpiece material at machining temperatures in excess of 800°C also contribute to the formation of groove wear. During the machining process, titanium molecules from the workpiece accumulate on the front of the insert and are "welded" to the cutting edge at high pressure and temperature, forming a chip tumor. When the tumor is removed from the cutting edge, it takes the carbide coating of the insert with it, so titanium machining requires special insert materials and geometries.
4. Tool structure suitable for titanium machining
The focal point of titanium machining is heat, and large amounts of high-pressure cutting fluid have to be injected onto the cutting edge in a timely and accurate manner in order to remove the heat quickly. There are unique configurations of milling cutters on the market that are specifically designed for titanium machining.
